1. Field of the Invention
This invention relates generally to bicycle derailleurs. More specifically, this invention relates to a derailleur design, which is suited for the front, driving sprocket assembly.
2. Description of the Related Art
Conventional bicycle front derailleurs typically use a parallelogram mounted cage to force the chain against the adjacent larger sprocket when an upshift is initiated. The combination of rotation and friction eventually forces the chain to climb the larger sprocket and engage the teeth. Conversely, axial pressure by the cage forces the chain to derail from the larger sprocket and drop on the smaller sprocket. Since these movements of the chain occur in the loaded section of the chain, the operator has to reduce the pedaling force in order to allow the chain to climb the side of the larger sprocket, as friction is not sufficient to overcome the downward pressure of the tensioned chain. This happens usually when it is least affordable—while climbing a gradient. Similarly, when the chain derails from a larger to a smaller sprocket, the operator has to reduce the pedaling force, or the chain will slam on the smaller sprocket. Additionally, when the chain is not aligned between the front and rear sprockets, as in some gear combinations, the chain tends to rub against the sides of the cage producing noise and causing wear. Another problem with the typical front derailleur is the requirement of precise calibration of the travel of the cage in response to input from the control unit—the shifter. Deviation from very strict parameters causes the chain either to fail shifting or drop from the sprockets. To mitigate this problem, shifters have been continuously improved, resulting in complex and costly apparatuses, which can work only with the corresponding front derailleurs. Numerous attempts have been made to resolve these issues. Most make incremental improvements to the typical front derailleur as described in U.S. Pat. No. 4,734,083 to Nagano. An attempt to overcome the above mentioned shortcomings has been made by Sam Patterson in his U.S. Pat. No. 5,649,877 granted on Jul. 22, 1997. Yet another attempt can be seen in U.S. Pat. No. 8,337,343 granted to Chang Hui Lin on Dec. 25, 2012. Shifting precision has also been approached by introducing electronic control and actuation as seen in U.S. Pat. App. No. 20130061705.
The enumerated attempts fail to address the main cause of the shortcomings—the fact that the derailleur is stationary along the travel path of the chain: the cause of friction.
A different direction in which attempts have been made to overcome the shifting shortcoming is by modifying the sprockets in order to facilitate the derailing of the chain. An example of this approach can be seen in Schmidt, et al. U.S. Pat. No. 5,738,603 from Apr. 14, 1998. Besides putting an onus on manufacturing and the corresponding high cost, this kind of modification might weaken the sprockets.
A more radical attempt to solve the whole number of above stated problems has been made by Bruce Browning—U.S. Pat. No. 5,205,794 from Apr. 27, 1993. A brief look at the invention, however, reveals excessive complexity, weight and elevated manufacturing costs. Also, the invention has a very limited range of speeds and the sectored, articulated sprockets render them fragile and prone to failure.